1. Field of the Invention
The present invention relates to semiconductor processing. Still more particularly, the present invention relates to a system, method and computed program product for monitoring the film thickness and trench depth in a semiconductor process.
2. Description of Related Art
Semiconductor processing techniques used e.g., for the fabrication of integrated circuits and microelectromechanical systems (MEMS) employ multiple processing steps aimed at creating or removing a film of material in a layer, or creating or removing selectively parts of layers to create topography on these layers. Examples include plasma etching and chemical vapor deposition processes.
The final result of these processes is often required to have a precisely controlled dimension such as the thickness of a film or the depth of a trench. An example is the excavation of trenches in silicon to make transistors by the Shallow Trench Isolation (STI) process. In this case, the incoming part is a silicon wafer which has had deposited on it a stack comprising multiple layers. The top layer is a mask, typically photoresist, which has openings in the form of trenches in it. During the STI process, the pattern of openings will be transferred to the silicon by etching trenches into the silicon, wherever there is an opening in the mask. During this step the mask itself may also be etched. It is desirable to control the final depth of the trench in the silicon to within a few nanometers. Because the trench is cut through the mask and all subsequent layers into the silicon, knowing the depth of the trench into the silicon requires knowledge of both the total trench depth and the thicknesses of all intermediate layers.
This high degree of precision is typically achieved by maintaining strict control of the thicknesses of the layers on the incoming wafer, and of the etch process itself. The prior art achieves the necessary degree of control by employing a complex multi-step approach. Once the etch process is considered stable, it is run on one or more test cases. The resulting wafer is then taken to a metrology station where the relevant thicknesses and depths are measured. The metrological techniques are typically Scanning Electron Microscopy (SEM) or Atomic Force Microscopy (AFM), which are destructive, or optical measurements. These measurements provide a calibration which allows the etch rates to be inferred. Then the necessary precision of the layer thickness or trench depth can be achieved simply by controlling the time of the etch step. During production etching, additional wafers are periodically pulled from production and measured using the techniques described above to ensure that the process remains in control. If necessary, either the etch time or etch rate is then adjusted to bring the thicknesses or depths back to the target.
Although this technique works well to achieve the necessary control, it has two undesirable aspects. First is the cost, in material, time and labor, to perform the calibration measurements. Second, this mode of operation requires that the process be maintained much more precisely than would otherwise be necessary.
Periodic measurements are then required to ensure that the necessary control is being maintained. If destructive measurement techniques are used, this entails an additional cost in the form of lost product. Finally, if control is lost, by the time the necessary measurements have been performed so that this becomes known, additional out-of-spec product will have been produced.
For this reason, it would be desirable to have a technique that allows the trench depths and layer thicknesses to be measured continuously, in-situ, on each wafer, during the production etch. In-situ measuring trench depths and layer thicknesses would reduce the need for off-line metrology steps, eliminate the production of out-of-spec product, and allow larger tolerances to be used for control of the process. Making the measurement in-situ, however, is more difficult than making it at a dedicated metrology station. Methods which require contact with the wafer, or are destructive, cannot be considered, so optical methods are preferred. Optical methods in the prior art which are compatible with in-situ real time measurement exist, but generally lack the sophistication to measure quantities on patterned device wafers of current interest.
For example, a method for measuring the thicknesses of layers in a multi-layer film stack is disclosed in Nishizawa, et al. in U.S. Pat. No. 5,587,792, which is incorporated herein by reference in its entirety. Nishizawa, et al. describe an apparatus for measuring the thickness of the layers of a thin semiconductor multi-layer film by irradiating the multi-layer semiconductor film with light having a wavelength range between visible and infrared light spectrum and a photometry system for continuous spectrometry of the reflected light from the multi-layer film, such as a Michelson interferometer. The interference waveform dispersion spectrum of light reflected from the multi-layer film is compared to a waveform obtained by numerical calculation using an optical characteristic matrix. Respective layer thickness values obtained from the calculated analysis of the spatial interference waveform are subjected to waveform fitting with actually measured values. The theoretical interference spectrum is recalculated while changing approximate values of the layer thicknesses until a match is obtained to obtain precise respective layer thicknesses.
The above described film thickness measurement technique requires that a homogeneous film stack exist throughout the area being measured. The features on modem device wafers are small and densely packed, so adapting this technique to such a wafer would require the use of a very small optical probe (i.e., illumination spot), and probably also the ability to translate the beam to locate the desired area for measurement. Doing so is difficult and impractical for an in-situ measurement.
Methods for measuring the depth of a trench are described by Kondo in U.S. Pat. No. 4,988,198, and Wickramasinghe in U.S. Pat. No. 5,392,118, each of which are incorporated herein by reference in there entirety. These methods exploit interference phenomena which occur when a light beam is partially reflected from both the top and bottom of a trench. Trench depth is inferred from the spacing of adjacent minima or maxima in the reflected light signal from the wafer. The reflection may be monitored as a function of time, in which case only relative depth information is obtained. It may also be monitored as wavelength or incidence angle is varied, in which case absolute depth information may be obtained.
These methods are also inadequate for problems like the STI case described above, because they only yield the total trench depth. They are also difficult to apply when the trenches are cut into a multilayered structure.
Recently there have been efforts to overcome these limitations by using more sophisticated algorithms in conjunction with optical reflectometry. The idea is to use a broad wavelength range and measure the spectral reflectivity over an extended spot on the wafer which includes two or more discrete regions, each with a different, possibly multi-layer, film stack. Surface topography on the wafer is accommodated by recognizing that the upper surfaces of the respective regions may not all lie in a single plane.
All of the prior art methods discussed above make use of the fact that the reflectivity of structures of the sort we are concerned with is determined by multiple interference effects. A light photon which has been reflected from the structure and then detected may be considered to have taken any of a large number of alternative paths. These paths may differ in having been reflected from different regions in the plane of the wafer, if these regions are separated by a distance less than the lateral coherence length of the light. Paths which undergo different combinations of reflections at the interfaces are also present, provided that their lengths differ by no more than the longitudinal coherence length of the light. The contributions from all of these paths add, and their relative phases determine whether they add destructively or constructively, hence the intensity of the observed signal. The phases are determined by the ratio of the path length difference to the wavelength. Where the interference is primarily constructive, the reflectivity is high, and where it is primarily destructive, it is low. This is the main way in which information about layer thicknesses and trench depths is embedded in the reflection spectrum. The magnitude of the reflectivity, and the amplitude of the variation of the magnitude from one wavelength to another, are primarily determined by the size of the refractive index discontinuity at the various interfaces and the relative sizes of the different regions—things which are incidental to the vertical dimensions of the structure which we are trying to monitor, although they are important if the reflectivity is to be matched by an optical model.
The methods rely on the use of an optical reflectivity model which is sufficiently detailed to account for each different area within the measured spot. The model takes the form of a function of several parameters. Each layer thickness and each trench depth within each discrete area is represented by a parameter in the model. In general there will be other parameters as well. The measurement of unknown layer thicknesses and trench depths is achieved by varying the values of the respective parameters until the difference between the observed spectrum and the model is minimized.
A general description of the method is disclosed by Solomon et al. in U.S. Pat. No. 5,900,633, which is incorporated herein by reference in its entirety. Thickness and composition of layers fabricated during manufacture can be determined using a measurement spot that is sufficiently large to irradiate areas of two or more different regions of the sample that result from its patterned features, generally at replicable locations. One or more of reflectance, transmittance, and radiance spectrance is measured, and the various parameters characterizing the thickness and composition in the patterned areas are obtained using, for example, a model-based analysis of the polarization and amplitude of the emanating radiation, the model parameters being iteratively adjusted to achieve a match with measured values. Measurements may be taken both before and also after treatment steps are effected, and/or by using measurements from the same location on designated samples undergoing the same process, to reduce the number of unknown parameters in a reference model, thus increasing the practicality and speed of the method.
Scheiner, et al. in U.S. Pat. No. 6,281,974 B1, disclose another description of substantially the same method described immediately above and is also incorporated herein by reference in its entirety. Scheiner, et al. state that the measuring method uses at least one desired parameter of a patterned structure having a plurality of features defined by a certain process of its manufacturing. The structure being represents a grid having at least one cycle formed of at least two locally adjacent elements having different optical properties in respect of an incident radiation. The method further employs an optical model which is based on at least some of the features of the structure and is capable of determining theoretical data representative of photometric intensities of light components of different wavelengths specularly reflected from the structure. The optical model also is capable of calculating the desired parameter of the structure. Essentially, a measurement area, which is substantially larger than a surface area of the structure defined by the grid cycle, is illuminated by an incident radiation of a preset substantially wide wavelength range. Light component substantially specularly reflected from the measurement area is detected, and measured data representative of photometric intensities of each wavelength within the wavelength range is obtained. The measured and theoretical data satisfies a predetermined condition. Upon detecting that the predetermined condition is satisfied, the desired parameter of the structure is calculated.
Another disclosure of a similar method is provided by Zalicki in U.S. Pat. No. 6,275,297, which is incorporated herein by reference in its entirety. The method disclosed by Zalicki is specifically intended for STI trench depth measurement. Zalicki describes measuring a depth geometry of a structure on a semiconductor substrate that includes a plurality of recessed and non-recessed portions, wherein one of the recessed and non-recessed portions includes a reference interface and one of the recessed and non-recessed portions has a dielectric layer thereon. The apparatus for measuring uses a broadband light source for irradiating the substrate and a detector for detecting a first spectral component comprising light reflected from the non-recessed portions, a second spectral component comprising light reflected from the recessed portions, and a third spectral component comprising light reflected from the dielectric layer. Spectral reflectance information of the detected rays is stored and a plot of reflectance intensity versus wavelength is generated. A depth geometry of one of the recessed portions and the dielectric layer are determined relative to the reference interface, based on an interferometric analysis of the plot, with the ability to distinguish depth geometries with a resolution as low as 100 angstroms. Zalicki further states that the method may be performed in-situ and that the analysis for determining the depth geometries preferably includes fitting the plot to a reflectance model.
With regard to the apparatus used to make the reflectivity measurement which forms the basis of these methods, the physical apparatus may take many forms, depending upon whether the measurement is to be made in-situ or in line, which wavelengths of light are to be employed, and many other factors. Descriptions of suitable arrangements are given by K. P. Kileen and W. G. Breiland (J. Electron Mater 23, 179 (1994), and Optical Diagnostics for Thin Film Processing by I. P. Herman, Academic Press (1996), p. 358), and by Perry, et al. in U.S. Pat. No. 6,160,621, for example, each of which are incorporated herein by reference in their entireties. It is a general requirement for each of these methods that the apparatus be capable of returning accurately the reflectivity of the surface being measured for a substantially broad range of wavelengths.
In each of these implementations, this method requires the construction of an optical model which is sufficiently complete so as to afford substantial agreement with the observed reflection spectrum when the appropriate values for the parameters are used. The model takes the form of an equation which includes parameters representing the quantities to be determined.
Standard minimization techniques are used to find the values of the parameters which produce the best agreement between the calculated model and the observed reflection spectrum. Agreement in this context means the minimization of a “merit function” defining a so-called “goodness of fit” between the measured and theoretical data. None of the prior art references mentioned above explicitly teaches how this merit function is to be defined, or how the minimization is to be done. However, the standard form of the merit function is the sum of the squares of the difference between the observed and calculated spectrum at some or all of the wavelengths for which measurements exist. Solomon et al. suggest that the Levenberg-Marquardt method can be used to perform the non-linear regression analysis (Press, W. H., Flannery, B. P., Teukolsky, S. A., Vetterling, W. T., Numerical Recipes, Cambridge University Press, 1992.) It can be problematic to apply such techniques to oscillatory data of the sort generated by these optical methods, however, because the merit function typically has many local minima which correspond to incorrect values for the parameters. These techniques require an initial guess for each of the parameters being determined, and there is always the danger that the algorithm will converge to a nearby local minimum rather than the global minimum which is the correct answer.
It is of key importance to the success of these methods that the merit function have a well-defined minimum, and that this minimum actually occur for the values of the parameters which correspond to the correct values of the thicknesses and depths being determined. If not, then it becomes likely that some combinations of incorrect values will yield values of the merit function which is nearly as low as, or even lower than, that of the correct values. With the prior art methods, this imposes the requirement that the optical model be capable of accurately reproducing the observed reflectance of the structure. In order to make this so, it is generally found to be necessary to include, in addition to the parameters representing the quantities to be measured, additional parameters representing other properties of the structure.
Examples of such parameters include:                1) The optical constants n and k of each layer in each area at each wavelength used;        2) The relative areas of the different discrete areas, Solomon, et. al. (col. 9, line 41), Scheiner et al. (col. 9, line 60, describing parameters C1, C2 and C3), and Zalicki (col. 7, line 37);        3) A parameter describing scattering from the sides of trenches, Zalicki (col. 7, line 37 describing parameter C4),        4) A parameter λ describing the coherence of the light in the optical system, Scheiner et al. (col. 7, line 35, describing parameter λ);        5) Heuristic “size coupling factors,” Scheiner et al. (col. 8, line 16, describing parameters c1 and c2);        6) Dissipation factors, Scheiner et al. (col. 8, line 43, describing parameters b2 and B); and        7) Polarization factors, Scheiner et al. (col. 9, line 10. describing parameters p1 and p2)        
These prior art methods are potentially suitable for in-situ metrology because a small illuminated spot is not required. However, the prior art methods mentioned above are generally limited in their usefulness for continuous in-situ monitoring due to several factors previously unaddressed in the prior art.
One shortcoming is that the prior art methods require an accurate measurement of the reflectivity of the wafer over a broad range of wavelengths. The reflectivity is the ratio of the incident to reflected optical power from the wafer. In an in-situ measurement, neither the beam incident on nor reflected from the wafer is directly accessible to measurement. Hence, the reflectivity has to be inferred from a measurement which is a convolution of the reflectivity and some other system properties, such as the transmission of a window. If these properties are unknown, or if they are subject to change, then the inference requires the addition of further parameters.
Another problem which the prior art has failed to fully address is minimization. The minimization problem which should be solved in order to make a measurement entails a search over a parameter space whose dimension is higher than the number of unknowns being determined. At minimum, there must be one fitted parameter for each of the quantities being determined. Because these methods depend on finding an accurate fit between the observed and calculated reflectivity, however, additional parameters, such as mentioned (e.g., parameters for describing: scattering from the sides of trenches; the coherence of the light in the optical system; heuristic “size coupling factors;” dissipation factors; and polarization factors), must in general be used. Each additional fitting parameter which is required raises the level of difficulty of the problem.
Finally, the prior art reliance on the requirement of a substantially accurate fit imposes the limitation that the model be specific to a narrow range of structures for which it is suitable. This makes it inconvenient to use in a production environment where structures of many different kinds are processed, since different forms of the model have to be used with different structures.